1. Field of the Invention
This invention relates to an energy harvesting device, and more particularly to an energy harvesting device using SSHI (synchronized switch harvesting on an inductor) techniques.
2. Description of the Related Art
Referring to FIGS. 1 and 2, two conventional energy harvesting devices 11, 11′ are disclosed in “Revisit of series-SSHI with comparisons to other interfacing circuits in piezoelectric energy harvesting” by I. C. Lien et al., Smart Materials and Structures, vol. 19, no. 12, pp. 125009, 2010.
The first conventional energy harvesting device 11 is coupled between a piezoelectric vibrating device 12 and a load 13. The piezoelectric vibrating device 12 vibrates at an operating frequency in response to a force applied thereto, and converts the vibration into an AC voltage. The first conventional energy harvesting device 11 converts the AC voltage from the piezoelectric vibrating device 12 into a DC voltage, and outputs the DC voltage to the load 13. The first conventional energy harvesting device 11 uses parallel-SSHI techniques, where a series connection of a switch 111 and an inductor 112 is coupled to the piezoelectric vibrating device 12 in parallel, and where the switch 111 operates in an ON state for a predetermined time period each time a vibration displacement of the piezoelectric vibrating device 12 reaches its extremum (i.e., a phase difference between a center time point of the predetermined time period and a respective time point at which the vibration displacement reaches its extremum is zero). As a result, energy harvested by the first conventional energy harvesting device 11 from the piezoelectric vibrating device 12 reaches its maximum when the operating frequency of the piezoelectric vibrating device 12 is equal to a short circuit resonant frequency of the same. However, the harvested energy decreases rapidly with deviation of the operating frequency from the short circuit resonant frequency.
The second conventional energy harvesting device 11′ differs from the first conventional energy harvesting device 11 in that the second conventional energy harvesting device 11′ uses series-SSHI techniques, where the series connection of the switch 111 and the inductor 112 is coupled to the piezoelectric vibrating device 12 in series, and where the switch 111 operates in the ON state for the predetermined time period each time the vibration displacement of the piezoelectric vibrating device 12 reaches its extremum (i.e., the phase difference between the center time point of the predetermined time period and the respective time point at which the vibration displacement reaches its extremum is zero). As a result, energy harvested by the second conventional energy harvesting device 11′ from the piezoelectric vibrating device 12 reaches its maximum when the operating frequency of the piezoelectric vibrating device 12 is equal to an open circuit resonant frequency of the same. However, the harvested energy decreases rapidly with deviation of the operating frequency from the open circuit resonant frequency.